As the popularity of liquid-filled hard capsules (LFHC) increases, formulators are becoming more interested in ways to evaluate the compatibility for their formulations with the capsule shell, particularly in the pharmaceutical arena, where it is sometimes necessary to use hygroscopic fill materials that can cause capsules to break. While breaks in capsules filled with powders can be a nuisance breakage of LFHCs is unacceptable since a single broken capsule can contaminate an entire package.
The theory behind capsule breakage is that hygroscopic fill materials pull water from the capsule shell. The shell then becomes brittle, making it less resistant to impact forces normally encountered during handling. While the inventors, during the course of their research identified the economic and commercial drawbacks of the discussed capsule breakage, they found that no systematic study has been performed to identify the causes of such breakage and identify methods to limit the waste.
Capsules consisting of telescopic parts have been known for a long time. U.S. Pat. No. 525,845 of 1894 describes a telescopic capsule, comprising a cap, having an annular constriction approximately in the middle and flares toward its open end. The capsule body is designed to be embraced by the annular constriction when the parts of the capsule are fitted together. This allegedly results in a good fit of the cap of the capsule on the body thereof.
In another capsule, such as is disclosed in U.S. Pat. No. 2,718,980, the capsule cap has on its inside an annular projection and an annular groove. The capsule body is also provided adjacent to its opening with an annular projection and an annular groove. A reliable seal between the cap and body of the capsule is allegedly ensured in that the projection and groove of one part of the capsule snap into the groove and projection of the other capsule part when these parts are pushed one into the other.
Both the capsule cap and the capsule body of the capsule described in the German Patent Specification 1,536,219 are formed with an annular constriction. When the two parts of the capsule are fitted one into the other, the convex annular bead formed on the inside of the capsule cap in conjunction with the constriction enters the annular constriction of the capsule body.
Capsules for containing medicaments are generally made today from hard gelatin in a dipping process. In this process, properly designed pins are dipped into an aqueous solution of gelatin and are subsequently withdrawn from the gelatin solution. When the gelatin has dried on the pin, the gelatin body is stripped from the pin and the resulting capsule part is cut to the desired length. In this practice it has been found that annular convex projections or concave recesses on the pin render the stripping of the gelatin body more difficult. Besides, it is almost impossible to obtain an airtight seal between the capsule cap and the rim of the capsule body when capsule parts are fitted together. This is due to the length tolerances of the capsule parts, particularly to the different distances between the rim and the annular recess of the capsule body. For a reliably fitting joint, the mating annular concave recesses or convex projections must interengage although this does not ensure an airtight seal and conventional wisdom has propagated the belief that the air-tight seal is mandatory for LFHC.
Therefore, early in the course of their investigations with LFHCs, the inventors recognized that the design of LFHC and the effect of hygroscopic fill materials was an important aspect to be considered in order to support the needs and uses required by LFHCs. As such a need exists to overcome the deficiencies of current LFHCs.